Whatever you do, practice a lot.
Generally speaking, if your technique is good, racking the slide is a little more reliable because you fully compress the spring. That means more energy to feed the round into the chamber. Not a lot more energy, but it can make a difference in some cases.
But that requires good technique which requires practice.
The other reason to rack the slide is if there's any reasonable chance you might have to operate a self-defense gun that either doesn't have a slide release or has one that is hard to operate. Some guns have very small, or hard to reach, slide releases and some don't have one at all. If you depend on a gun like that for self-defense you should practice racking the slide, not using the release even if the gun you're shooting at the range has a release.
Using the slide release will usually be faster if the release is easy to operate and easy to reach. If all the guns you use for self-defense are like that and, when using the slide release, will ALWAYS feed the first round from a completely full magazine even when the gun is dirty, then use the slide release.